1. Field of the Invention
The present invention relates to an image reading apparatus for use with an image recording apparatus, such as a copying machine, a facsimile, or a printer, and more particularly to a preheating system for the image reader.
2. Description of the Prior Art
In an apparatus for converting image information on an original into electrical signals, such as a facsimile, a printer, an image data storage apparatus, or a copying machine, a charge-coupled device (CCD) line sensor has been predominantly used as an image reading apparatus.
There are two known types of CCD line sensors. In the first type of CCD line sensor, a plurality of CCD "short line" sensors are arrayed linearly or in a zig-zag fashion, forming a CCD "long line" sensor to read an image of a large original. In reading the image, the CCD image sensor comes into close contact with the original and slides over the original. In the second type of CCD line sensor, a reduction optical system reduces a projected image of the original, and focuses the reduced image onto a small CCD line sensor.
In conventional use, after the power is turned on, the CCD line sensor is continuously supplied with electric power, and placed in a driven state. The only control applied to the CCD line sensor is for outputting the image data as read.
The continuous power supply and the constantly driven state stabilize the CCD line sensor. The resultant image data suffers from little variation in optical density and, hence, is stable. However, when the CCD line sensor is continuously supplied with electric power and is ceaselessly driven, it is heated to a high temperature. If adequate cooling by a heat sink is not provided, the lifetime of the sensor is unavoidably reduced. In this respect, the reliability of a typical CCD line sensor is poor.
In addition, the continuous power supply unnecessarily consumes electric power.
To save electric power and prevent excessive temperature rise, the generation of a drive clock signal for image formation may be stopped while image reading is not taking place. That period of time is referred to as non-read time.
In one such proposal found in Japanese Patent Unexamined Publication No. 63-196158 for stopping the drive clock signal generation during the non-read time, reading of image data may be inconsistent. More specifically, when the non-read time is extensive, the image sensor will cool down. If the read operation starts while the image sensor is cool, an image is read and printed cut by a machine whose characteristics are still unstable. Particularly in successive printing, the printed copies from one original image may have different optical densities. In this respect, the reliability is also unsatisfactory.